Frequency-modulated carriersignal translating system



I las ATTORNEY INVENTORy DETECTOR FIG.I.

FIG.2.

R. L. FREEMAN Filed March 1, 1940 BANDv-PASS SELECTOR FREQUENCY-MODULATED CARRIER-SIGNAL TRANSLATING SYSTEM Sept. 30`

LEZ.

FREQUENCY CHANGER FREQUENCY Y CHANtERv FREQUENCY CHANGER i TUNABLE OSCILLATOR FREQUENCY CHANGER TUNABLE OSCILLATOR AMPLIFIER ILMBE R FRE UENCYIQ- AMPLIFIER ws "ifa rhgl

Sept. 30, 1941. R, L, FREEMAN l 2,257,092

'FREQUENCY-MODULATED CARRIER-SIGNAL TRANSLATING SYSTEM Filed March 1,l 1940 2 she'et's-sneet 2 50 -o V y o- FREQUENCY NPUT CHANGER 74 50@- 73 I 52 OUTPUT vTUNABLE 69 osclLLA-roR sELEcToR A sprrn 9 l 7s? T J INVENTOR BERT L. FREEMAN ATTORNEY.

- thereto.

Patented Sept. 30, 1941 oNlTizo STATES PATENT omer FREQUENCY-MODULATED CARRIER- SIGNAL TRANSLATNG SYSTEM Robert L. Freeman, Flushing, N. Y., assignor to y Hazeltine Corporation, a corporation of 'Dela- AppumionMarcn 1, '1940, serial No. 321,610

(ci. 25o- 20) 13 Claims.

This inventionrelates generally to frequencymodulated carrier-signal translating systems and particularly to systems of -such type including signal-translating channels wherein at least one practice, a frequency-modulated carriersignal having a small frequency deviation is first generated and then the frequency deviation of the frequency-modulated carrier signal is increased before radiation; while in a frequency-modulated carrier-signal receiver; it is often desirable to convert the received frequency-modulated carrier signal to one having a diiferent type of modulation, for example, amplitude modulation, that is substantially entirely free of frequencymodulation components. Further, in the reception of frequency-modulated carrier signals, it is desirable to effect such conversion as early as possible in the carrier-signal translating channel to avoid the use of wide band selectors which are necessary in conventional frequency-modulated carrier-signal receivers to accommodateV the relativelywide frequency-modulationsidebands. Thus, narrow band selectors having a high impedance and consequently a greater gain when included in intermediate-frequency amplifiers may be used in the signal-translating channel after the frequency-modulated carrier signal has been converted to an amplitude-modulated carrier signal, the selectivity being provided primarily by the converter itself or by one or more stages of the signal-translating channel prior Furthermore, in the usual ltype of frequencymodulated carrier-signal receiver, it is customary to convert the frequency-modulated signal to an amplitude-modulated signal before detection, since conventional signal detectors respond only to amplitude modulation.- Such conversion systems 'are known and are sometimes designated hybridizers or hybridization systemsv in view of the fact that an applied frequency-modulated carrier signal is converted to a hybrid signal having the same frequency modulation and, in addition, amplitude modulation corresponding to the frequency modulation. For example, hybridizafrequency-response characteristic of which has a finite constant slope other than zero. in the pass band accommodating the frequency-modulated carri'er-'signai vand its sidebands. Thus, even after. the frequency-modulated carrier signal is once hybridized, it is still necessary that subsequent carrier-signal translating networks Vbe capable of `passing a band broad .enough to accommodate the frequency-modulation component sidebands otherwise severe distortion will arise in the amplitude-modulation components. 0n the other hand, if only the amplitude-modulation component sidebands are present, subsequent vcarrier-signal translating networks capable of passing a relatively narrow band may be used.

Since phase modulation dilers from modulation only in the magnitude of the amplitude of sidebands corresponding to a given modulation-frequency signal. it is to be understood that the term frequency modulation is used hereinafter to include not only frequency modulation in the rigorous sense but also phase modulation. v

In addition, there exists at present the need for a converter attachment for use with conventional amplitude-modulated carrier-signal receivers by means Aof which the receiver may be adapted for the .reception of frequency-modulated carrier signals. In such a frequency-modulation converting apparatus of this nature, as well as in frequency-modulated carrier-signal receivers, per se, it is highly desirable that the converted modulated-carrier signal be substantially entirely free of frequency-modulation components.

It is an object of the present invention, therefore, to provide a frequency-modulated carriersignal translating system which is not subject to the aforesaid disadvantages and undesirable features of the arrangements of theprior art.

It is a more specific object of the invention to providev a frequency-modulated carrier-signal translating channel elective to alter at least one of the characteristics of Va frequency-modulated carrier signal translated thereby. l

It is another object of the invention to provide i. an improved frequency-modulated carrier-signal transmitting means.

`It is another object of the invention to pro- It is a further object of the invention to provide improved means for converting a frequencytion' is effected'in any network, the transmission 55 modulated carrier signal to one having 'a differfrequency enttypeofmodulatlonwhlchlsentirelyfreeof f modulationcomponents.

It isan additional-object of the invention to provide improved means' for converting a frequency-modulated signal to an ampia, tuile-modulated can'ierxsignal in a carrierquency ctn-a frequency-modulated receiving Inaccordancewiththeinventiomafrequencymodulated carrier-signal translating channel comprises an input circuit adaptedV to have applied thereto a frequency-modulated.iliputcar-l rler signal and-means ,coupled to the invutclr-I cuit for selectively deriving directly from the@`4`l input 'signal at least two separate carrier. signals each having frequency-modulation com- .ponents corresponding to those of the input tr;

signal. The channel also includes means for gaseous vcomplete frequency-modulated carrier-signal' receiver embodying the invention; Pig. 3 is a similar diagram of a frequency-amplitude modulation converterattachment for conventional am- ."plitude-modulated carrier-signal receivers; Pig.

combining the derived carrier signals in substantially the same time relationship to develop an output Acarrier-'signal having modulation components' vdependent on the frequency-modulation componentsl of' both of the derived carrier signais. andl 'an output o circuit for the output signal nllpledftothe combining means@ n l p ,v :More speciiically in, accordance with the invention, a frequency-modulated vcarrier-signal translatingl vchannel particularly i suitable for use in a transmitter comprisesfrequency-changing 1 `means for; modulating'A a frequency-modulated carrier-signal input with a 'constant frequency oscillation v,to derive any intermediate-frequency frequency-modulated carrier the frequency o f which is equal tothe sumy of the con-- stant frequency and the carrier frequency. The

channel also includes bandpass selector means for. selecting such sum-intermedlate-frequency frequency-modulated "casujrierv signal and means for. modulating suchvselected carrier signal with the input frequency-modulated carrier signal to derive han. output frequency-modulated lcarrier signal, the frequency of which is thesum of the `sum-intermediate-carrier frequency andthe input carrier frequency, and the frequency deviationgoi whichis twicefthat' ofthe input signal,

andaba'nd-pass selector means for selecting such output frequency-modulated carrier signal.

Further, more specically in accordance with theinventicn,^ ra frequency-modulated carrier- `signal .translating channel particularly suitable ,fori usein a receivercomprises means for selectively-'deriving "from, a `received frequency-modulated carrier signal at least two separate carrier signals, onehaving the frequency of said 'input signal and the other having a frequency equal to the diilerencekof said input-signal frequency and a substantially constant frequency and effectively comprising "an intermediate-frequency frequency-modulated signal. The v.channel also includes means forselecting andconverting one of the derived signals.. preferably the intermediate-frequency signal, to develop a hybrid signal carrier having amplitude-modulation components corresponding 'to the frequency-.modulation components of said input signal, and

.means for combining the hybrid carrier signal' A with another' of said derived carriernsignals to develop an amplitude-modulated carrier signal having a carrier frequency equal to said sulo-- stantiallyconstant. frequency and fromv which the frequency-modulation components kare substanti-lilly balanced out. v

As used herein, the term frequency-modulated. carrier signal derived from a received signal" is intended to include the received carrier swhlle'j'Flg. 5 is av and current relationships in a balanced hy- ;bridi'zer# e' Aparticularly to ingsthere is shown a carrier-signal translating 4 is-'as'imilar diagram of a modification of the invention comprised', in a'balanced hybridizer: phase diagrem of the voltage Fig'. 1 of the dswi channel forfa frequency-modulated carrier-signal transmitter including an input circuit having terminalsl i0, i0 adapted to have applied thereto a frequency-madrilatedV carrier v Signal. Connected to the input terminals III, I0 is a mcdulator ory frequency changer vIl to which is coupledja tunable oscillator I2, there being coupled Ito the frequency changer l I in cascade, in the order named, a band-pass selector'll having a uniform transmission frequency-response characteristic 'over the frequency-modulation sidebands of the translatedsisnal and a second modulator or frequency changer I l which is coupled also to the input terminals I 0, I0 by way of a band-pass translating circuit'which may comprise simply conductors I5, I5. A band-pass selector iivhaving a uniform transmission frequency-response characteristic over the frequency-modulation sidebands of the translated signal is coupled'tolthe frequency lchanger' Il and is provided with an terminals Il, vIl. It is to be noted that the width of the frequency-modulation sidebands of the selector i8 is twicev the signal translatedby that of the input signal.

In considering the operation ofthe frequencyu modulatedEcarrier-signal translating channel of Fig. l. which operates to .double the frequencymodulation deviation of'an applied frequencymodulated signal, it will be input terminals I0, l0 are coupled toa conventional source offrequency-modulated carrier signais u. while the output terminals l1, I'l are coupled eventually to a conventional antenna through one or more similar channels, if desired. The frequency-modulated carrier-signal wave supplied to the frequency changer Il is l modulated by a constant-frequency oscillation voltage a from the oscillator l2 to derive an intermedlate-frequency carrier signal (m4-wo), the l carrier frequency of which is thesum of the constant frequency of the oscillator I 2 and the carrier frequency of the input signal and having f frequency-modulation components corresponding to those ofthe input signal. Such intermediatefrequency signalis lselected by the band-pass selector I3' and modulated in the frequency changer I! by a frequency-modulated carrier signal o, derived from thereceiv'ed signal andl translated over the band-pass circuit l5, to

output circuit rhaving assumed that lthe develop a frequency-modulated earner-signal output (2m-kut) the carrier frequency of which is the sum of the sum-intermediate-carrier frequency and the input carrier frequency' and has frequency-modulationA components dependent upon those of both of the derived carrier signais but has twice the frequency-modulation deviation of the input signal. This output wave is selected by the band-pass selector `Il for translation through the output terminals II, l1 to an antenna for radiation. Stated in other words, the arrangement on Fig. l: comprises means for directly deriving a plurality of separate carrier signals from the input signal, each having frequency-modulation components cor-- Vresponding to those of the input signal. Specifically, the units il, l2 and I3 comprise means and the circuit Il comprisesmeans for directly deriving a separate carrie signal (u.) froml the input' signal, each derived signal having modulation components corresponding to those of the input signal. The two signals so derived are thereafter combined in substantially the same time relationship in frequency changer It to upon those of veach of the above-mentioned derived signals.

The fact that the frequency-modulation deviation of the signal output of frequency changer I4 of Fig. 1 selected by band-pass selector I6' is twice that of the signal input can be readily appreciated from the fact that the frequency deviations of the sum-intermediate-carrier-frequency signal output of'frequency changer li andthe signal input are'in phase; that is, the carrier frequencies-of these two signals swing in frequency inrunison. Therefore, the sum-frequency signal output of frequency changer M undergoes a frequency swing ytwice that of the signal input'. case the difference-intermediate-frequency output of frequency changer Il is selected by selector l 3 and combined with the signal input in `frequency changer Il and the difference-frequency output is selected by selector I.; In this case also the carrier-frequency deviations of the two signals combined 'in frequency changer Il' are in phase 'and the frequency deviation of the difference-frequency output is double that of the input signal. It is only necessary that simi- -lar heterodyne product signals be derived from Ashown schematically a complete frequency-modulated carrier-signal receiver comprising coupled in cascade, inthe order named, an antenna system 2|, 2l, a tunable radio-frequency amplifier 22 includingone or more stages of ampli- `A similar result isprocured in` for directly deriving a am carrier signal (Wl-oo) the output of radio-frequency amplifier 22 by way of a band-pass translating circuit which may comprise simply conductors 21, 21, an amplitude-modulation detector 28, an audio-frequency amplifier 29 including one or more stages of amplification, and a signal-reproducing device.30. The selector and hybridizer may comprise a resistor 3| of high value, which may be comprised in whole or in part ofthe anode resistance of the preceding vacuum tube, a shunt inductance 32, and shunt capacitance 33, the latter being shown in dotted lines because it may be comprised in whole or in part of the inherent shunt capacitance of the circuit. The values ofthese circuit constants preferably are proportioned so that the frequency-response characteristic of the network has a uniform slope of substantial value over the frequency-modulation sideband range so that the input fre-g` quency-modulated carrier signal is converted into a hybrid signal.f

In considering the operation of the receive of Fig.l 2, a frequency-modulated carrier signal u.

`intercepted by the antenna lsystem 2i, 2l is .l derive the output carrier signal (2w+wo) vwhich has frequency-modulation components dependent selected and amplified in the radio-frequency amplifier 22 and this amplified frequency-modulated carrier signal is modulated in frequency changer 23 by a substantially constant frequency oscillation ou from oscillator 24 to derive an intermediate-frequency frequency-modulated signal (oi-o0) having frequency-modulation Vcomponents corresponding to those ofthe received' signal. This derived signall is selected andconverted to a hybrid signal including both frequency-modulation and amplitude-modulation components bythe selector and hybridizer V2li. This hybrid signal is applied to the frequency changer 26 and mixed therein with a frequencymodulated carrier signal u, derived from the receivedV signal through the band-pass signaltranslating circuit 21, 2I and having frequencymodulation components corresponding to those o1' the received signal, so that the frequency-modulation components are substantially demodulated to derive an amplitude-modulated signal o that is substantially freel of frequency-modulation components. In other words, the means 23, 2l, 25 for selecting and supplying the signal (ai-ue) to frequency changer 26 has a transmission frequency-response characteristic which is different from that of the means 21 for selecting andsupplying the signal (u.) to the frequency changer 26. The resulting pure amplimde-modulated constant frequency'canier signal a is detected flcation, a modulator or'frequency changer 23 having: a tunable oscillator 2l coupled thereto, a selector and hybridiaer 25. a modulator or frequency changer 23 which is' coupled also to in a conventional manner by detector 2l, amphiled by audio-frequency amplifier 23, and reproduced by the reproducing device 3l.

A' simplified explanation of the operation of the receiver of Fig. 2 may be obtained by considering a frequency-modulated carrier Signal. the carrier frequency u. of which swings between 40 and 41 megacycles. Thus, if the local oscillator frequency is 30 megacycles, the intermediatecarrier frequency (in-po) Vswings between l0 and 1l megaeycles. The carrier-frequency output uc of the frequency changer 26 is equal to the difference between the input carrier frequency and the intermediate-carrier frequency and is thus 30 megaoycles regardless of the frequency swing of the input signal. Y However, since 'the hybridizer 2l has a frequency-responsel characteristic of uniform slopeover the frequency-mod- -ulation sidebands, the 3 0megacyc1e output varies in amplitude in accordance with the .instantaneous deviation of the frequency-modulated carrier signal. f

In its simplest form, the selector and hybridizer unit 25 comprises the circuit illustrated. The intermediatev frequency is small compared to the carrier frequency of the input signal and the circuit constants of the unit 25 are so proportioned that the circuit appears as only an inductive reactance at the intermediate-carrier fre-` quency, resonates `at a frequency somewhat greater than the intermediate-carrier frequency, and appears as a negligible capacitive reactance at frequencies of the order of that of the input carrier signal and at higher frequencies. It will beapparent that the frequency-modulation components are eliminated in the output of the frequency changer 26 since the two frequency-modulation signal inputs thereto have frequency devi- .ations which are in phase; thatis, their carrier frequencies swing in unison over equal deviation ranges. Thus, the instantaneous difference frequency of these two signals is constant; that is,

the frequency deviations of the two signals eifectively cancel each other. Again a similar result may be procured by selecting the sum-intermediate-frequency output of frequency changer 23 heterodyning it with the input signal in frequency changer 26.

. acuosa vquency of the oscillator 25 and coupled to an out` put including terminals 33, 3l.

In considering the operation-of the frequencymodulated carrier-signal converter of Fig. 3, the amplified first intermediate-frequency frequencymodulated carrier signalk (wi-uo) selected -by the band-pass selector 34 is heterodyned in frequency changer 26 with a constant frequency oscillation to derive sum and difference second and third Aintermediate-frequency frequency-modulated signals (m-wo-i-wnz) and (w-oo-uoz). respectively,

' having frequency-modulation components corre- Referring to Fig. 3 of the drawings, there is shown a converter for converting a frequencymodulated carriersignal to an amplitude-modulated carrier signal substantially free of all fre'- quency-modulation components for subsequent reception by conventional amplitude-modulated carrier-signal receivers, elements similar to those of the receiver of Fig. 2 being designated by similar reference numerals. The converter attachment of Fig..3 differs primarily from the receiver of Fig. 2 in that the frequency changer 23 has coupled. thereto a rst intermediate-frequency band-pass selector 34 in place of the hybridizer sponding to those of the received signal, the latter of which is hybridized by selector 43, 44, 45, 46 to derive a hybrid carrier signal including both amplitude-modulation and frequency-modulation components. 'I'he development of such hybridsignalmay be explained by the fact that,v

with two coupled circuits tuned to the saine resonant frequency, the vector sum of the voltages across the two circuits maximizes at some frequency different than their mean resonant frequency and the effective response of the selector has a steeper and more nearly uniform slope than can 4be procured from the response characlteristic of a simple double-tuned transformer.

This vector sum voltage is obtained by effectively connecting the two tuned circuits .43, 44 and 45, 46 in series in an input circuit'cf frequency changer 31. The effect of such a vuniformly slop-v ing response characteristic over the frequencymodulation band is similar to that of a conventional frequency discriminator in converting frequency deviations of an applied signal'to amplitude variations without, at the same time, affecting the frequency-modulation deviations. These two signals are combined in the frequency chang- 25 and the frequency changer 26 has coupled thereto a local oscillator 35. Further, the output circuit of frequency changer 26 includes a hybridizing band-pass selector network 36 which is coupled to a frequency changer. 31 to the output circuit of which are coupled circuit terminals 38, 38 for connection to a conventional amplitude-modulated carrier-signal receiver.

selector and hybridizer consisting of a tuned primary circuit 43, 44 inductively coupled to a tuned secondary circuit 45, '46 and having a transmission characteristic of uniform slope over the pass band corresponding to a third intermediate-frequency frequency-modulation sideband associated with the difference-interniediate-carrier frequency. In order to Aderive a hybrid signal having amplitude-modulation as well as frequency-modulation components, the signal voltage across the primary circuit 43, 44 is added, to that across secondary circuit 45, 46 through blocking condenser !8 and applied to another control electrode of the frequency-changer tube 31. 'I'he output circuit of the frequency changer 31- includes a selector 41, 46 tuned to twice the freer 31 and, due to the unlike frequency-response' characteristics' of the selector 39, 40, 4|, 42 and the selector and hybridizer 43, 44, 45, 46, the frequency changerl 31 'is effective to produce an amplitude-modulated carrierA signal 2o@ of twice the oscillator frequency. The manner in which the frequency-modulation components are eliminated from the output signal is similar to that described above in connection with `the system of Fig. 2.

For example, if the first intermediate-frel quency frequency-modulated signal translated by the band-pass selector 3 4 instantaneously swings, .from 1.9 to 2.1 megacycles and back again and the frequency of the local oscillator 35 is 270 kilocycles, the output of the frequency changer I 26 includes frequency-modulated signals which instantaneously swing from 2.17 to 2.37 -megacycles and 1.63 to 1.83 megacycles and back again. The instantaneous frequency deviations of these two signals from their mean carrier frequencies are in the same direction and of the same magnitud'e. Hence, when they are selected and one is hybridized to obtain a hybrid signal having amplitude modulation as well as frequency modulation and the two are combined in the frequency changer 31, the difference-frequency signal is of constant frequency but pure amplitudemodulated. This amplitude-modulated carriersignal output has a carrier frequency twice the oscillator frequency or, in the example given, 540 kilocycles, which is of a suitable frequency to be applied to a conventional amplitude-modulated signal receiver.

The circuit of Fig. 3,'as has been pointed out, is effective in converting frequency-modulated signals to amplitude-modulated signals becausel the selector 33, 40, 4I', 42, which is tuned to the that, if these two signal-translating vcircuits have both like transmissionV frequency-amplitude .response characteristics and like transmission frequency-phase response characteristics, the output signal at terminals 38, 38 is completely devoid of any form of modulation. On/the other hand, if these two signal-translatingdevices have like transmission frequency-amplitude response characteristics but unlike transmission frequency-phase response characteristics, as by givingy both selectors constant times of transmis.. sion with frequency but different absolute times of transmission. an input frequency-modulated carrier signal will develop an 'output signal at terminals 38, 38which is solely phase-modulated. An advantage of the circuit of Fig. 3 over that of Fig. 2 resides in the fact that both of the signais applied tothe frequency changer 31 are amplified to a relatively high signal level so that the amplitude-modulated signal derived lfrom frequency changer 31 is considerably greater in amplitude. Furthermore, the selector circuits between-the two frequency changers 26, 31 contribute to the over-all selectivity of the receiver. If two tuned circuits are used in each selector, as shown, the over-all selectivity is increased by four circuits over that obtainable with the arrangement of Fig. 2. The frequency changer 23 which precedes the band-pass Iselector 3B is primarily -effective to reduce the relatively high frequency of the received frequency-modulated signals and thus improve the selectivity obtainable by the use of a low first intermediate frequency as well as to provide amplification. Referring particularly to Fig. 4 of the drawings, there is represented asignal-translating channel for converting a frequency-modulated carrier signal to an amplitude-modulated carrier signal by means of a balanced hybridizer. This term may be defined as a circuit arrangement for use in a frequency-modulation to amplitude-modulation conversion system to balance out noise or residual amplitude modulation, such as transmitter hum, of a frequency-modulated carrierl signal when the frequency modulation is substantially zero. The translating channel of Fig. 4 comprises an input circuit having terminais 50, 50 adapted to have applied thereto a frequency-modulated vcarrier signal. Coupled to the input circuit is a modulator or frequency changer 5| which is also coupled to a tunable oscillator 52. The output circuit of the frequency changer 5| includes in series a resonant circuit 53, 54 tuned to the sum-intermediate-carrier frequency thereof and a resonant circuit 55, 55

tuned to the difference-intermediate-carrier fre quency thereof. A phase-shifting circuit including an inductance 51.and a resistor 58 is connected across the tuned circuit 53, 54 and the junction of inductance 51 and resistor 58 is conby way of a conductor 63 and a parallel resonant circuit 54 tuned to the difference-intermediatecarrier frequency of vthe frequency changer 5| The tuned circuits 50, 64 are inductively coupled in opposite phase toinductance 55. The frequency changers 65 and 66 are connected Vin push-pull relation to provide a common output circuitincluding a parallel connected condenser 51 and an inductance 68 tuned to twice the frequency'of the oscillator 52. Inductively coupled to the inductance 58 is an inductance B9 for supplying va second harmonic oscillation voltage from the oscillator l52 to the` output of the system by way of a selector 10 tuned to twice the oscillator frequency, a phase shifter 1|, and conductors 12,'12. -An output circuit including terminals 13, 13 is inductively coupled to inductance 68 by an inductance 14.

In considering the operation of the translating channel of Fig; 4, it is assumed that the input terminals 50, 5|) are coupled to a frequency-modulated carrier-signal source. Such carrier signal is heterodyned with a constant frequency voltage from oscillator 52 to produce sum and difference frequency current components in the output circuit of frequency changer 5| having frequency-modulation components corresponding to those of the received signal. The explanation may be facilitated by the diagram of Fig. 5 which is a quasi-vector or phase diagram showing the relation between the instantaneous phases of the several quantities relative to the same arbitrary reference point in their respective cycles, a positive horizontal vector representing `zero phase angle relative to such reference point.

Furthermore, the instant at which the several quantities is represented is that at which the input carrier frequency is passing through its mean frequency. l'.I'he instantaneous phases of the sum and dierence frequency currents are represented by vectors I+ and I- in the phase diagram of Fig. 5. The difference-frequency current produces a. voltage across tuned circuit. 55, 56 which is represented by the vector E5s,as and voltages across tuned circuits 50 and 54. which are represented respectively by vectors E-so and Eher. The vector sum E-as of voltages E-sass and E-eo represents the total difference-frequency voltage applied'to the control grid of tube 55,

while 'the vector sum E-ss of voltages' E-sass' and Eer represents the total difference-frequency voltage applied to the control grid of tube 65.

When the instantaneous frequency of the frequency-modulated difference-frequency signal is other than the mean frequency, the vector relationships are altered. In general, at such frequency the voltage vector Ess is lengthened while at the same instant the voltage vector E-ses4 is shortened. At another frequency on the other side of the mean frequency, vector Ess is shortened 'and E-e is lengthened. Thus, during frequency deviations of the difference-frequency signal, the voltages represented by vectors Eyes and E-en vary inamplitude in opposite directions, that is, they represent hybrid difference- V frequency voltages having their amplitude-modulation factors in opposite phase.A

The sum-frequency current component I+ .develops a voltage E+5a,54 across tuned circuit 53, 54, a voltage E+e5 across resistor 52 which is a part of the phase-'shifting circuit 5|, 62, and a voltage E+et across resistor 58 which is a part of the phase-shifting circuit 51, 58. Ei'es and E+es represent the net 'sum-frequency voltages applied to the control grids of tubes 65 and 55, respectively.

The sum-frequency voltage vectors, as shown in Fig. represent conditions when the frequencymodulated sum-frequency carrier .signal is P655- ing through its mean frequency. By appropriately damping tuned circuit 53, 54, the sum-frequency voltage vectors remain constant in amplitude over a frequency band including the maximum frequency deviations of the frequency- Y modulated sum-frequency carrier signal.

The application of voltages E-u and E+ to tube i6 produces by heterodyne detection a plate current component I of twice oscillator frequency which is amplitude-modulated and substantially free of frequency-modulation components. Similarly the application of voltages E-ss and E+ to tube 65 produces a plate current component 'Iss which is also amplitude-modulated and substantially free of frequency-modulation components. When :the circuit is so proportioned that the phase angle 0- between the sum- -frequency current I+ and the voltage E+ss is equal ,to the phase angle a+ between the difference-frequency current and the voltage E-ss at zero frequency modulation of the sum-frequency and difference-frequency signals and the scalar product of E-e and E+ is equal to the scalar. product of E-as and E+ss, the currents les and Iss t are equal and in phase. Hence, by reason of the carrier component, the amplitude l*modulation of which is in correspondence with the frequency modulation of the input signal. A noise free and unmodulated carrier may be re-supplied by selecting the second harmonic of oscillator S2 by selector 10, correcting its phase appropriately byphase-shiftingcircuit 'Il and coupling it to the output through inductance 69,' tuned circuit 61, 68, and inductance 14 lto enable the resultant ed in a conventional manner.

While there have been described what are at present `considered to be the preferred embodiments of this invention, it will'be obvious to those skilled in the art that various changes and modications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A frequency-modulated carrier-signal translating channel comprising, an input circuit adapted to have applied thereto a frequencymodulated input carrier signal, means coupled to said input circuit for selectively deriving directly from' said input signal at least two separate carrier signals each having frequency-modulation components corresponding to those of said input signal, means for combining said derived carrier signals in substantially the same time relationship to develop an output carrier signal having modulation components dependent upon the frequency-modulation components of both of said lating channel comprising,

derived carrier signals, and an output circuit for said output signal coupled to s aid last-nam means.

2. A' frequency-modulated carrier-signal translating channel comprising, an input circuit adapted to have applied thereto a frequencymodulated input carrier signal, means coupled to said input circuit for selectivelyderiving directly from said input'signal at least two sepa- 10 rate carrier signals of different frequencies each having frequency-modulation components corresponding to those of said input'signal, means for combining said derived carrier signals to develop an output carrier signal having modul5 lation components dependent upon the frequencymodulation components of both of said derived -carrier signals, and an output circuit for said output signal coupled to said last-named means.

3. A frequency-modulated carrier-signal transaninput circuit adapted to have. applied thereto a frequencymodulated input carrier signal, means coupled to said input circuit for selectively deriving from said input signal two separatek carrier signals4 one having the frequency of said input signal and the other having a frequency equal to that of a heterodyne product of said input signal and a substantially constant frequency Aoscilla-l tion and each having frequency-modulation4 components corresponding to those of said input signal, means hfor combiningsaid derived carrier signals to develop an output carrier signal having modulation components dependent upon the frequency-modulation components of both said de- 4vrived carrier signals, and an output circuit for said output signal coupled to said last-named means. t v I 4. A frequency-modulated carrier-signal translating channel comprising, an input circuit adapted to have applied thereto a frequencymodulated input carrier signal, means coupled to said'input circuit for selectively deriving from said input signal two separate carrierA signals one having the frequency of said input signal'` and the other having a frequency equal to that of a heterodyne product of said input signal and a substantially constant frequency oscillation and each having frequency-modulated components corresponding to those of said input signal,

means yfor combining said derived carrier signals amplitude-modulated carrier signal to be detectto develop a frequency-modulated loutput signal having a frequency equal to that of a heterodyne product of the frequencies of said derived carrier signals and an increased frequency-modulation deviation, and an output circuit for said voutput signal vcoupled-to said last-named means.

, 5. A frequency-modulated carrier-signal translating channel comprising, an input circuit adapted to have applied thereto a frequency 50 modulated input carrier signal, frequency-chang ing means coupled to said input circuit for modulating said input signal with a constant frequency oscillation to derive an intermediate-frequency frequency-modulated carrier signal the frequency of which is equal to that of a heterodyne product ing said intermediate-frequency signal with the input signal to derive an output frequency-modulated carrier signal the frequency of which is equal to that of a similar heterodyne product of said intermediate-frequency signal and the input signal and the frequency deviation of 'whichlis 2,257,o92 l a 7 substantially greater than that of said input signal, and band-pass selector means for selecting said output frequency-modulated carrier signal.

6. A frequency-modulated carrier-signal translating channel comprising, an input circuit adapted to have applied thereto a frequencymodulated input carrier signal, means coupled to said input circuit for selectively deriving from said input signal a plurality of separate carrier signals each'having frequency-modulation components corresponding to those of said input signal, means for selecting at least two of said derived carrier signals .comprising band-pass translating circuits having different transmission frequency-response characteristics over their respective pass bands, means for combining said last-named carrier signals to develop an output signal which depends upon the frequency-modulation components of both of said derived carrier signals,-and an output circuit for said output signal coupled to said last-named means.

7. A frequency-modulated carrier signal translatlng channel comprising, an input circuit adapted to havel applied thereto a frequencymodulated input carrier signal, means coupled to said input circuit for selectively deriving from said input signal a plurality of separate carrier signals each having frequency-modulation components corresponding to those of said inputv signal, means for selecting at least two of said derived carrier signals comprising selectors having different transmission frequency-phase response characteristics, means for combining said last-named carrier signals to develop a phasemodulated output signal from which the frequency-modulation components are substantially balanced out, and an output circuit for said output signal coupled to said last-named means.

8. A frequency-modulated carrier-signal translating channel comprising an input circuit adapted to have applied thereto a frequencymodulated input carrier signal, means coupled to said input circuit ior selectively deriving from said input signal a plurality o f separate carrier signals each having frequency-modulation components corresponding to those of said input' signal, means for selecting at least two of said derived carrier signals comprising band-pass translating circuits having different transmission frequency-amplitude response characteristics, means for combining said last-named carrler signals to develop an amplitude-modulated output signal from which the frequency-modulation components are substantially balanced out, and an output circuit for said output signal coupled to said last-named means.

9. A frequency-modulated carrier-signal translating channel comprising, an input circuit adapted to have applied thereto a frequencymodulated input carrier signal, means coupled to said input circuit 'for selectively deriving from said input signal Vat least two separate carrier signals one having the frequency of said input signal and the other having a frequency equal to the difference of said input signal frequency and a substantially constant frequency and each having frequency-modulation components corresponding to those of said input signal, means for selecting and converting at least one of said derived carrier signals to a hybrid carrier signal including amplitude-modul-ation components corresponding to the frequency-modulation components of said input signal, means for combining said hybrid carrier signal with another of frequency equal to said substantially constanty frequency and from which the frequency-modulation components are substantially balanced out, and an output circuit for said output signal coupled to said last-namedmeans.

10. 'A frequency modulated carrier signal translating channel comprising, an input circuit adapted to have applied thereto a frequencymodulated input carrier signal, means coupled to said input circuit for selectively deriving from said input signal 'at least two separate carrier signals one having a frequency equal-to the sum of said input signal frequency and a substantially constant frequency and the otherhaving a frequency equal to the difference of said input signal frequency and said substantially constant frequency and each having frequency-modulation components corresponding to thoseof said input signal, means for vselecting and converting at least one of said derived carrier signals to a hybrid carrier signal including amplitudemodulation components corresponding to the frequency-modulation components of said input signal, means for combining said hybrid carrier with the other of said derived carrier signals to Y develop an amplitude-modulated output signal having a carrier frequency equal4 to twice said substantially constant frequency and Vfrom which the frequency-modulation components are substantially balanced out, and, an output circuit for said output signal coupled to said last-named means.

1l. A frequency modulated carrier signal translating channel comprising, an input circuit adapted to have applied thereto a frequencymodulated input carrier signal having undesired modulation components which may include signal and noise amplitude-modulation components, means coupled to said input circuit for selectively deriving a plurality of separate carrier signals each having frequency-modulation Vcomponents and said undesired modulation components corresponding to those of said input signal, means for selecting at least two of said derived carrier signals comprising band-pass translating circuits having different transmission frequency-response characteristics, means for combining said derived carrier signals to develop an output signal from which thel frequency-modulation components and said undesired modulation components are `translating channel comprising, an input circuit adapted to have applied thereto a frequencymodulated input carrier signal `having undesired modulation components which may include signal and noise amplitude-modulation components, means coupled to said input circuit for selectively deriving a plurality of separate carrier signals each having frequency-modulation Acomponents and said undesired modulation components corresponding to those of said input signal, means for selecting at least two of said derived carrier signals comprising selectors having different transmission frequency-amplitude response characterlstics, means for combining said last-named carrier signals to vdevelop an output signal from which the frequency-modulation componentsj 8 i and said undesired modulation components and thev carrier are substantially balanced out and which contains amplitude-modulation components corresponding only to the frequency-modulation components of said input signaL'an out- 4 put circuit for said output signal coupled to said last-named means, and means coupled to said output circuit for supplying a carrier, for said amplitude-modulation components.

13. A frequency modulated carrier signal signals each having 4frequency-modulation components and said undesired modulation compo-V nents corresponding to those o! said input signal.

means for selecting at least two of said derived vcarrier signals comprising selectors having "dii- Yferent transmission `frequency-amplitude re-y signal from which the frequency-modulation sponse characteristics, means for combining said last-named carrier signals to develop an output componentsland said undesired modulation components and the carrier are substantially balanced out and which contains amplitude-modulation components corresponding only to the frequency-modulation components of said input signal, an output circuit for said output signal coupled to said last-named means, and meams coupled to said output circuit for supplying a carrier-for said amplitude-modulation components including means coupled to said oscillator for deriving a carrier the frequency of which is a harmonic of the constant frequency'of said- ROBERT L. FREEMAN. 

